Hydraulic pressing device and method for operating the same

ABSTRACT

The invention relates to a method for operating a hydraulic pressuring device with a stationary part ( 26 ) and a moving part ( 24 ), whereby the moving past ( 24 ) is moved relative to the stationary part ( 26 ) until a predetermined pressure is reached. In order to operate a hydraulic pressuring device of this type, for example a cable shoe (connecting) or riveting tool pressing device, whose handling is especially improved, the invention provides that moving part ( 24 ) returns automatically and completely to the initial position thereof when a predetermined pressure is reached.

The invention relates in the first place to a method for operating ahydraulic pressing device having a stationary part and a moving part,the moving part being displaced in relation to the stationary part untila predetermined pressure is reached.

Hand-operated or motor-driven hydraulic tools are often employed forcertain joining procedures, such as for example the pressing-on of cableeyes onto electrical conductors, or for riveted connections. These toolsare provided with an excess pressure valve which limits the oilpressure, and thus the compressive force of the moving part against theworkpiece to be pressed, to a maximum value. In order to ensure awell-made joint, e.g. of a cable eye to an electrical conductor, it isknown for the excess pressure valve to act only when a prescribedminimum compressive force is reached. This makes sure that the fullrequired compressive force was effective. After release of the excesspressure valve, the pressing device, or rather the moving part thereof,is returned manually to the initial position, i.e. the open position.

Having regard to the above-described state of the art, the invention isaddressed to the technical problem of providing a method for operating ahydraulic pressing device of the type under discussion, such as forexample a cable eye (connector) pressing device, or a riveting toolpressing device, in which method the handling aspect is especiallyimproved.

This problem is solved in the first place and to a substantial extent bythe subject matter of claim 1, it being provided that the moving partreturns automatically and completely back into its initial position,released by the predetermined pressure being reached. Accordingly, onachievement of a maximum pressure, there occurs an automatic opening ofthe pressing device and complete return of the moving part into theinitial position. The user is spared having to intervene manually inorder to open and return the moving part. The user is simultaneouslygiven an optical signal also, by the return movement of the moving part,that the joint has been properly made with the prescribed maximumpressure.

The invention relates moreover to a method for operating a hydraulicpressing device, such as for example a pipe clamping tool, that has astationary part and a moving part and an automatically actuating returnvalve, the moving part being biased into its initial position by meansof a return spring. In this type of hydraulic pressing device,especially a pipe clamping tool, it is known for the return stroke ofthe hydraulic piston to take place automatically once the switch-offpressure has been reached. Only for emergencies is an additional,manually operated, return valve provided. In normal operation, thepressing or the joint formation can only be ended after the maximumpressure has been exceeded and the tool opens by the automaticallyreturning hydraulic piston which carries the moving part with it. Knownconstructional solutions of the desired manner of operation consist ofusing an excess pressure valve which, after release, is arrested by amechanical stop mechanism and thus permits complete return travel of thespring-loaded piston. On renewed actuation of the pressing device, forexample when the motor is switched on, the arresting is mechanicallydisconnected and the excess pressure valve falls back into the closedposition. In order to provide a hydraulic pressing device of the generaltype here under discussion, such as for example a pipe clamping device,which is characterised in particular by an advantageous arrangement fromthe handling aspect, it is provided that the return valve is held openby the force of the return spring, and returns automatically to itsinitial closed position after removal of the restoring force. As aresult of this method according to the invention, no mechanicalcomponents are required for arrest of the piston or the moving part inthe initial position, which furthermore obviates structural solutionsfor disconnecting the arresting when the pressing device is in use. Inthe method according to the invention, the restoring force, which ispresent anyway, of the return spring, is advantageously used forreturning the moving part, so as to keep the return valve open over theentire return stroke of the moving part. No further arrest means areneeded. After completion of the return movement, there is no furtherhydraulic pressure, owing to a limiting abutment of the spring-biasedhydraulic piston, which results automatically in a return displacementof the return valve into the initial, closed position. The limitingabutment of the hydraulic piston also gives rise, shortly before an endposition, to the smallest hydraulic pressure effective to keep thereturn valve open.

The invention relates further to a hydraulic pressing device with astationary part and a moving part, the moving part being displacedrelative to the stationary part by a hydraulic piston and being movableback to an initial position by means of a return spring, the returndisplacement being releasable in dependence on a predetermined pressureby actuation of a return valve. In order to provide a hydraulic pressingdevice of the kind under discussion with improved functional reliabilityand handling properties, it is proposed that the automatically actingreturn valve be retained in the open position, throughout the entirereturn stroke of the hydraulic piston, by the pressure of the returningoil. Mechanical arrest of the return valve during the return stroke ofthe hydraulic piston or of the moving part can be dispensed with bymeans of this arrangement, which offers special advantages in operation.Known structural solutions consist, for example, in the use of an excesspressure valve which, after actuation, is arrested by a mechanical stopmechanism and thus makes possible a full reverse stroke of thespring-loaded piston. On a fresh actuation of the pressing tool, forexample by switching the motor on, the arresting is mechanicallydisconnected and the excess pressure valve goes back into the closedcondition. But in accordance with the invention, the pressure of thereturning oil, which is present anyway, after actuation of the returnvalve, is used for maintaining the return valve in the open position.The return valve acts automatically when a predetermined oil pressure isexceeded. The pressure of the returning oil, which decreases during thereturn of the hydraulic piston, is sufficiently high over the entirereturn stroke to keep the return valve in the open condition. It isfound especially advantageous in this connection for the return valve tobe formed as a valve piston, a partial piston surface area, effective inthe closed condition, being calculated having regard to the maximumpressure. To this end, the return valve consists for preference of avalve piston having for example a needle point which closes off a boreconnecting with the pressure space. The smaller partial piston surfaceeffective by reason of the bore diameter is engaged by the oil in thecourse of pressing by the hydraulic pressing device. If the oil pressureexceeds a value predefined by the bore diameter, the valve piston of thereturn valve is raised from its sealing seat by way of the partialpiston surface, whereupon a substantially greater piston area comes intoeffect. The return valve in this position operates with a substantiallylower limiting pressure than in the closed condition. The limitingpressure in this position is no longer defined by the smaller partialpiston surface area, but rather by the total surface area of the valvepiston, formed, as it is, as a longitudinally sliding piston. As anexample, a ratio of 400:1 can exist between the total piston area and asmaller, partial piston surface area which co-operates with the sealingseat. In consequence, the limiting pressure in the open position of thevalve piston is 400 times smaller than the actuation pressure in theseated position, i.e. in the initial closed condition. As a result ofthis arrangement, a return valve is provided which has high hysteresis,so that the valve piston remains in the open position throughout theentire return stroke of the hydraulic piston because of the oil pressureacting on the valve piston, despite the fact that the oil pressurecontinuously diminishes in the course of the return displacement. Thevalve piston only falls back into the initial closed position when theoil pressure falls below a prescribed minimum. This very low oilpressure equates to the fully returned position of the hydraulic piston.In an exemplary embodiment, actuation, i.e. the opening of the returnvalve, can occur at 600 bar, and automatic return travel thereof intothe initial position at 1.5 bar. In a hydraulic pressing device of thekind under discussion, in which the return valve is biased into theclosed position by means of a compression spring, it is providedfurthermore that the cylinder in which the valve piston is accommodated,has a discharge port to an oil reservoir, and that the discharge port isopened in the course of a displacement of the valve piston into the openposition. By this arrangement according to the invention, when theactuation pressure, defined by the force of the compression spring andthe smaller, partial piston surface area, has been exceeded, the oildoes not flow away directly into the oil reservoir. Rather the oilreaches the oil reservoir only through the discharge port, and thedischarge port is not opened until the valve piston has been displacedto a overlapping extent. The valve piston, to this end, can be fittedwithout much play, so that relatively little oil can escape by flowingpast it. In order to give the return valve controllable damping, it isprovided in an advantageous development of the invention that a reliefbore is provided to the rear of the valve piston, the bore penetratingthe cylinder wall. This bore serves for pressure relief of the spacebehind the piston, and the damping of the valve can be controlled by wayof the size and situation of the relief bore. The desired automaticreturn of the hydraulic piston is made possible, in simply-actingcylinders having a return spring, by making this return spring of suchdimensions that by pressing on the hydraulic piston, it creates an oilpressure which lies above the limiting pressure of the return valve inits longitudinal sliding condition. By this means, the valve is heldopen and the hydraulic piston returned. The arrangement of the returnspring is preferably such that the hydraulic piston returns all the wayto an abutment stop. In this end position, the return flow ceases andthe valve piston descends into its initial position, the seatedposition, after which the hydraulic pressing device is ready for thenext working cycle. As an example, a target maximum pressure of 600 barin the pressure space may be desired. If this is exceeded, the returnvalve is actuated and the limiting pressure sinks to about 1.5 bar. Therating of the return spring is, for instance, such that the pressure inthe pressure space is always 2.5 bar during the return stroke of thehydraulic piston. The pressure difference of at least 1 bar is mainlyabsorbed as a throttle loss in the flow through the small bore of thesealing seat, which bore co-operates, in the closed position, with thesmaller, partial piston surface; this pressure difference determines thethroughflow of oil and thereby the return velocity of the hydraulicpiston. One advantage of the return valve described is that along withthe excess pressure valve, which has to be provided anyway, noadditional parts, such as for example mechanical latching elements, arenecessary. Moreover the valve goes automatically, without necessity formanual unlatching, back to its initial state. In a further embodiment,it is provided that the valve piston can be displaced into an openposition by hand. Such hand operation is desired for instance forinterrupting the pressing procedure. To this end, the valve piston ismoved into an open position, whereby the discharge port to the oilreservoir is opened. This results in a fall in the oil pressure andthereby a return displacement of the hydraulic piston. A pulling part isfurthermore of advantage, connected to the valve piston and passingthrough the cylinder. This pulling part, in a preferred embodiment, ismovable by hand, by means of an actuating rocker. This actuating rockerconstitutes for the user an advantageous lever arm, by means of whichthe valve piston can be lifted up from the valve seating against theforce of the compression spring which acts on the seating. Thisarrangement in accordance with the invention ensures at all times amanual return stroke of the hydraulic piston, necessary in emergencies.In a further embodiment, it is provided that the valve piston ispot-shaped on its rear side relative to the surface of the valve pistonwhich is exposed to pressure. It is furthermore proposed that thepulling part comprises a drive head, which is in engagement with a drivenose on the valve piston. On tripping the actuating rocker, the valvepiston is accordingly displaced away from the valve seating by means ofthe pulling part which has the drive head. The compression spring, whichco-operates in defining the actuation pressure, can here bear directlyon the pot-shaped valve piston. An embodiment is preferred, however, inwhich the compression spring acts on the valve piston by way of thepulling part. A further advantage, especially in assembly or repairoperations, arises from the drive nose being a spring washer disposed inthe pot wall of the valve piston. An alternative provision has the drivenose integrally formed as a radial collar on the pot wall of the valvepiston, preferably the inner wall thereof, the collar acting at the sametime to centre the pulling part within the valve piston. Furthermore,the bore which co-operates with the smaller piston surface of the valvepiston can be provided in a seating disc screwed-in into the cylinder.Resulting from this arrangement, it is possible in very simple manner tochange the seating disc and the valve piston, the piston beingassociated with the pulling part by way of the spring washer. A furtherpossibility is for the other end of the compression spring which acts onthe valve piston to be supported against a screw, by which the desiredpreloading of the compression spring can be adjusted. Adjustment of thelimiting pressure is thereby enabled. It is provided in addition thatthe drive head is formed as a circumferential flange on the cylindricalpulling part. It proves advantageous, moreover, especially with a viewto a high functional reliability, for the pulling part to act on thevalve piston by way of a spigot portion of reduced diameter located atthe centre of the drive head. By these means, in very simple manner,there is achieved a moment-free transfer of force from the pulling partto the valve piston. Alternatively or in combination with theabove-described embodiments, it can also be provided that the drivehead, in the open condition of the return valve, defines an activepiston surface. From this, it follows that the oil flowing in, after thevalve is opened, acts directly on the pulling part, the drive head ofthe pulling part thus constituting the piston surface or at least a partthereof. Here it is preferred that the drive head, in the open conditionof the return valve, constitutes a partial piston surface integratedinto the active valve piston surface of the valve piston. As a result ofthis arrangement, the drive head, or the active piston surface thereof,is an integral component of the total piston surface, it being preferredfurthermore that the valve piston surface, which is now formed to beannular, is flush with the piston surface of the drive head, at least inthe operating condition. In this connection, an arrangement is preferredin which the valve piston is a hollow cylinder having a circularcross-section, the resulting annular front surface constituting thevalve piston surface. It is provided in a further variation of thisembodiment that the drive head provides, in the closed condition, apartial piston surface, the area of which is calculated with referenceto the desired maximum pressure. To this end, the drive head ispreferably equipped with a closure member, e.g. a needle point, whichcloses off a bore communicating with the pressure space. If the oilpressure exceeds a value predefined by the bore diameter, the drive headof the pulling part, and with it the valve piston, are lifted from thesealing seat by the partial piston surface formed by the needle point,whereupon the substantially greater piston surface provided by the valvepiston and the drive head comes into action. It is furthermore proposedthat the diameter of the discharge port be smaller than the height of aclosed circumference of the valve piston. For preference in thisconnection, the diameter of the discharge port is smaller than theheight of a closed circumference of the valve piston which faces thevalve piston surface, so that the discharge port is initially opened tothe extent of a crack and that only after the valve piston is firstlifted. It is additionally proposed that, to the rear of the valvepiston surface, an annular groove, open to the outer pot sleeve, beprovided. This groove stands preferably at least partly in communicationwith the discharge port in the closed condition of the valve. Thearrangement is chosen in such a way that, with raising of the valvepiston, there occurs opening of the discharge port for outflow of theoil, while at the same time the open annular groove is cut off from thedischarge port. This is achieved by making the distance between thevalve piston surface and the annular groove greater than the diameter ofthe discharge port. A further provision is that an axially aligned flowpassage extends from the valve piston surface, to connect the valvepiston surface with the annular groove. This flow passage serves in thefirst place to permit outflow of the unavoidable oil residues in thestationary, closed operating condition, without the prior occurrencethereby of a pressure rise in the remaining gap. In the second place,the flow passage is kept so small that when the valve is opened,displacement of the piston takes place, since in that way the outflowingoil likewise leads to a closure of the flow passage, because of thequantity. In this, there proves to be a significant advantage in that,by reason of the connection of the valve piston surfaces with therearwardly disposed annular groove which takes up residual quantities ofoil, the oil pressure in the course of the valve closing procedure dropsoff rapidly, which leads in consequence to a more rapid closing of thevalve. In this connection, it is further provided that the diameter ofthe flow passage is smaller than the diameter of an oil inlet bore ofthe valve. It is also proposed that the annular groove be formed in theouter wall of the pot, for a substantially horizontal disposition of thecylinder bore. An alternative proposal is that the annular groove isformed in the cylinder bore, and that the valve piston has anassociated, radial bore. The latter is in communication with the valvepiston surface by way of the axially aligned flow passage. The annulargroove formed in the bore of the cylinder is for preference provided atthe level of the discharge port. Furthermore, it is conceivable for boththe cylinder bore and the exterior wall of the pot to be each equippedwith a respective annular groove for the uptake of oil residues.

As an alternative or in combination, it is also conceivable to permitonly partial return travel of the hydraulic piston. In this case, therating of the return spring is such that its force in a particularposition within the working stroke of the hydraulic cylinder is nolonger sufficient to keep the return valve open.

The invention is described in greater detail below, with reference tothe accompanying drawing, which illustrates exemplary embodiments only,and in which:

FIG. 1 shows a schematic sectional view of a pressing device accordingto the invention, equipped with a return valve, in the course of apressing operation;

FIG. 2 shows a view corresponding to FIG. 1, but in the course of areturn stroke of a hydraulic piston of the pressing device;

FIG. 3 shows a schematic cross-sectional view of a motor-driven drivenhydraulic pressing device, in the initial position;

FIG. 4 shows an enlarged cross-section from FIG. 3, but in the course ofa pressing operation;

FIG. 5 shows a section taken along the line V—V in FIG. 4;

FIG. 6 shows a sectional view corresponding to FIG. 5, but in the courseof the return stroke of the hydraulic piston of the device;

FIG. 7 shows a sectional detail view of a return valve in a furtherembodiment;

FIG. 8 shows a view corresponding to FIG. 7, but relating to a furtherembodiment of the return valve

First with reference to FIG. 1, there is illustrated and described areturn valve 1, e.g. for a hydraulic pressing device 2. This returnvalve 1 can find application in either hand-operated or motor-drivenhydraulic tools.

The return valve 1 consists essentially of a valve piston 3 with aneedle point 4, centrally disposed at the front end and tapering to apoint, to form a partial piston surface (effective valve seatingsurface) substantially smaller than the total piston surface 5 anddefined by the diameter of a bore 7 communicating with a pressure space6. The bore 7 is closed off by the needle point 4 in an initial closedcondition as illustrated in FIG. 1.

To its rear, the valve piston 3 is acted on by a compression spring 8,whereby the needle point 4 is pressed against the bore 7 with a forcewhich contributes to the determination of a maximum release pressure. Inthis way, there is substantially provided a pressure-limiting valve ofthe seating constructional type.

In order to actuate the pressing device 2, for example to press cableeyes or connectors onto electrical conductors, or also for rivetconnection or pipe pressing, oil is pumped into the pressure space 6 byhand or motor power. The resulting rise in oil pressure displaces ahydraulic piston 9 disposed in the pressure space 6 against the force ofa return spring 10 in the direction of the workpiece to be pressed (seearrow a in FIG. 1).

In order to assure a proper connection, actuation of the return valve 1is aimed at in every pressing, thereby guaranteeing that the fullpressing force was effective. Thus it is provided, for example, thatwith a maximum pressure of 600 bar acting on the hydraulic pistonsurface 9′, the return valve 1 opens. This maximum pressure is definedby the very small partial piston surface of the needle point 4,projected onto the bore 7, or for that matter by the cross-sectionalarea of the bore 7 and by the pressing force of the compression spring 8on the valve piston 3.

Now if the oil pressure exceeds the predefined maximum value of forexample 600 bar, the valve piston 3 is displaced out of its sealingseating on the bore 7 against the force of the compression spring 8, andthen, all at once, a substantially greater piston surface area, namelythe entire piston surface 5 of the valve piston 3, comes into action. Byvirtue of the rearward displacement of the valve piston 3, a dischargeport 12 provided in the cylinder 11 which accommodates the valve piston3 is at least partially uncovered, for the return flow of the oil intothe oil reservoir 13 (see arrow x in FIG. 2). The valve piston 3 can befitted in with little play, so that relatively little oil can flow awaypast it through a relief port 14 into the oil reservoir 13.

In this position, illustrated in FIG. 2, otherwise the longitudinalslide valve position, the return valve 1 again functions as a pressurelimiting valve, but now in the longitudinal slide valve constructionalmode with a substantially lower limiting pressure, since the latter isnow here defined by the substantially greater piston area of the valvepiston 3. Accordingly, in the exemplary embodiment shown, a diameterratio of 1:400 exists between the smaller effective partial pistonsurface (needle point 4 in bore 7) and the total piston surface area 5,which has the result that the limiting pressure in the open position ofthe valve according to FIG. 2 is 400 times less than the releasepressure.

At the rear of the valve piston 3, moreover, there is provided therelief bore 14 already mentioned, which passes through the wall of thecylinder 11 in the direction of the oil reservoir 13. The bore 14 servesfor the relief of pressure on the rear side of the piston. What is more,the damping of the return valve 1 can be influenced by the size andposition of the bore 14.

A desired automatic return stroke of the hydraulic piston 9 is madepossible through the agency of the return spring 10, in that the springis so dimensioned that by bearing on the hydraulic piston 9, it producesan oil pressure in the pressure space 6, which pressure is above thelimiting pressure of the return valve 1 in its longitudinal slide valveposition according to FIG. 2. By this means, the return valve 1 is keptopen and the hydraulic piston 9 reversed (see arrow b in FIG. 2).

As a rule, the rating of the return spring 10 is such that the hydraulicpiston 9 moves completely back to the stop. In this end position, thereturn flow of oil ceases, which effects a descent of the valve piston 3into its initial closed position. The pressing device 2 is then readyfor the next operating cycle without further mechanical preparations,such as for example the disconnecting of a mechanical arrestingarrangement.

It is also possible to let the hydraulic piston 9 return only part of astroke. For this, the rating of the return spring 10 is such that, at apredetermined position within the working stroke of the hydraulic piston9, its force is no longer sufficient to keep the return valve open.

In the embodiment shown, a limitation of the oil pressure in thepressure space 6 to 600 bar is desired. If this is exceeded, the returnvalve 1 acts, and the limiting pressure drops to about 1.5 bar, becauseof the ratio of the areas, 1:400, of the piston surfaces one to theother. The rating of the return spring 10 here is such that the pressurein the pressure space 6 always has a value of at least 2.5 bar duringthe return stroke of the hydraulic piston 10. The pressure difference ofat least 1 bar is mainly taken up as a throttle loss in the flow throughthe small bore 7; it determines the oil throughput and thereby thevelocity of the return stroke of the hydraulic piston 9.

The advantage of this kind of return valve 1 is that, apart from theexcess pressure valve which has to be provided anyway, no additionalparts, such as for example mechanical latching elements, are needed. Inaddition, the valve 1 returns automatically to its initial conditionagain, without manual unlatching being necessary, following a dip belowthe limiting pressure that holds the valve piston 3 open, as in FIG. 1.

In FIG. 3, an electric motor-driven hand pressing device 2 isillustrated, with a return valve as previously described. A pressingdevice 2 of this type is known, for example, from the German PatentApplication having the File Number 197 31 054, not previously published.The content of this patent application is hereby incorporated as to itsfull content into the disclosure of the present invention, also for thepurpose of incorporating features of this patent application in claimsof the present invention.

In this pressing device 2, an electric motor 15 is provided, which has astep-down gearbox 16. The latter, through a shaft 17, drives aneccentric 18, which in turn, by way of a roller bearing 19, acts on ahigh-pressure delivery piston 20.

The drive of the electric motor 15 is effected by a battery, or by anaccumulator 22 integrated in a handle 21.

When a finger-actuatable switch 23 is actuated, oil is pumped from theoil reservoir 13 into the pressure space 6. By this means, the hydraulicpiston 9 is displaced in the direction of its end working position,against the action of its return spring 10, taking with it a moving part24 of the head 25 of a pressing device. The head further comprises astationary part 26, relative to which the moving part 24 can move.

The return stroke of the hydraulic piston 9 takes place by virtue of thereturn spring 10, as soon as the return valve 1 opens by reason of thepredetermined maximum pressure being exceeded—as described previously.

In this exemplary embodiment, the rear of the valve piston 3 of thereturn valve 1 is formed to be pot-shaped, i.e. facing away from itspiston surface 5. A pulling part 27, aligned axially with respect to thevalve piston 3, protrudes into the pot interior, with a drive head 28which is formed by a circumferential flange on the cylindrical pullingpart 27. In contrast to the previously described exemplary embodiment,the bore 7 which co-operates with the needle point 4 of the valve piston3 is not provided directly in the housing of the device but in ascrewed-in seating disc 29. This has advantages, especially in regard tomanufacture. Further, the sealing seat can thus also be replaced in verysimple manner when required.

Valve piston 3 and seating disc 29 are disposed in a transverse bore 30in the pump cylinder 31 and centred thereby. In order to hold the valvepiston 3 in a positively-locked manner on the pulling part 27, anexpanding ring 33 is disposed within the pot wall 32 to the rear of thedrive head 28. The compression spring 8 acts on the valve piston 3 inthe direction of the seating disc 29 by way of the pulling part 27 inthe region of a spigot portion 40 of reduced diameter centrally disposedon the drive head 28. A moment-free transfer of force from the pullingpart 27 to the valve piston 3 is effected in very simple manner by theloading of the valve piston 3 through the small spigot portion 40.

At its other end, the compression spring 8 is supported on a base 34 ofa likewise pot-shaped threaded body 35 which is screwed into the bore30.

The body 35 is axially penetrated by the pulling part 27; at the freeend of the pulling part 27 which projects from the pump cylinder 31, alever arm 36 of an actuating rocker 37 is hingedly connected. The rocker37 is supported on the external surface of the pump cylinder 31approximately midway along the length of the rocker by a curved portion38 of part arcuate shape in cross-section and the rocker defines at itsfree end an outwardly directed actuating key 39.

In an initial, closed condition of the return valve 1, as shown in FIG.5, the opening 7 is closed off by the needle point 4 of the valve piston3. In this position, moreover, the discharge port 12, which is directedto the oil reservoir 13, is covered over by the pot wall 32 of the valvepiston 3.

The piston 3 in this initial, closed condition, is at an axial spacingfrom the threaded body 35, in order to assure an axial displacement ofthe valve piston 3 when the maximum pressure is exceeded. At the levelof the gap which is thus present between valve piston 3 and threadedbody 35, the relief bore 14, which likewise leads to the oil reservoir13, is positioned.

As in the manner detailed with reference to the exemplary embodimentpreviously described, when a predefined maximum pressure in the pressurespace 6 is exceeded, the valve piston 3 is lifted against the force ofthe compression spring 8, upon which the discharge port 12 is opened forreturn flow of the oil into the oil reservoir 13. Because of theresulting pressure drop, the hydraulic piston 9 is displaced back againinto its initial position by the agency of its return spring 10 (arrow din FIG. 6).

When there is a drop in pressure below the limiting value defined by thepiston surface area 5 and by the force of the compression spring 8, thevalve piston 3 falls back automatically into its initial, closedposition.

Furthermore, the valve piston 3 which is formed to be pot-shaped can belifted by means of the outwardly extending pulling part 27, by way ofthe actuation rocker 37, by pressing on the actuation key 39. In thisway, a manual return stroke of the hydraulic piston 9, required inemergencies, is ensured at all times.

The threaded body 35 serves further for setting the desired preloadingof the compression spring, and thus for adjusting the limiting pressure.

An alternative embodiment of the return valve 1 is illustrated in FIG.7. In contrast to the previously described embodiment, the valve piston3 of the return valve 1 is formed not to be pot-shaped but rather as ahollow cylinder with substantially constant thickness of the pot wall32. The valve piston 3 thus formed is penetrated by the pulling part 27,aligned axially with respect to the piston 3, the plate-like drive head28 of the pulling part 27 being gripped from behind by a radial collar40 of the valve piston 3, which forms a drive nose. The collar 40projects from the pot wall 32 into the interior of the valve piston 3,the radial extent of this collar 40 being dimensioned such as to effectat the same time a centering of the pulling part 27.

Considered in the axial direction of the valve piston 3, the radialcollar 40 is offset from the piston surface 5 by the magnitude of thethickness of the drive head.

The compression spring 8 which encircles the pulling part 27 issupported at one of its ends on the base 34 of the threaded body 35 and,at its other end, on the rear face of the radial collar 40 of the valvepiston 3, for engaging the valve piston 3 and also, through the radialcollar 40, for engaging the pulling part 27 in the closed position ofthe valve.

By the described arrangement of the radial collar 40, the drive head 28of the pulling part 27 is set into the piston surface 5 of the valvepiston 3. The drive head 28 thus forms, in the open condition of thereturn valve 1, a partial piston surface 41 integrated into the pistonsurface 5. At the centre of this partial piston surface 41, the needlepoint 4 is formed, which in the closed position closes off the bore 7 ofthe return valve 1 and accordingly is now a component of the pullingpart 27. This needle point 4, in cooperation with the bore 7 in theclosed condition, defines a partial piston surface, the area of which iscalculated with reference to the desired maximum pressure.

To the rear of the total valve piston surface formed by the pistonsurface 5 and by the partial piston surface 41 of the pulling part 27,there is provided an annular groove 42, open to the outer surface of thepot; in the exemplary embodiment of FIG. 7, this groove is formed in theouter wall 32 of the pot. This annular groove 42 is radially outwardlyclosed off by the bore wall of the pump cylinder 31. Only in the regionof the discharge port 12 is there an overlap, in the closed condition ofthe return valve 1, so that the annular groove 42 communicates with thisdischarge port 12 by way of a gap 43 created by the overlap.

The distance b from the annular groove 42 to the piston surface 5 is forthis reason greater than the diameter c of the discharge port 12. Itfollows from this that, in the closed condition of the return valve 1, aclosed periphery of the valve piston 3 closes the discharge port 12 offfrom the gap formed between piston surface 5 and seating disc 29.

The annular groove 42 communicates with the valve piston surface 5through an axially aligned through passage 44. The diameter of thisthrough passage 44 is here kept small. In the exemplary embodimentshown, the diameter of the through passage corresponds to about half thediameter of the oil inlet bore 7.

An opposite disposition of the through passage bore 44 relative to thedischarge port 12 is preferred, moreover.

When a predefined maximum pressure in the pressure space 6 is exceeded,the drive head 28 of the pulling part 27 is lifted up and, by it, thevalve piston 3 is lifted up against the force of the compression spring8, whereupon the discharge port 12 is opened for return of the oil intothe oil reservoir 13. At the same time the annular groove 42 is movedout of the region of the outlet port 12 by this upward displacement ofthe valve piston 3, giving a complete radial closing off of this annulargroove 42.

The through passage 44 mentioned serves in stationary closed operationto allow unavoidable oil residues to drain off into the annular groove42, which residues can flow away through the discharge port 12 by way ofthe gap 42 provided, and this without the occurrence of a rise inpressure in the remaining gap between piston surface 5 and seating disc29. The through passage 44 is nevertheless kept so small that when thereturn valve 1 is opened, the valve piston 3 is displaced smoothly intothe open position, since the oil which then enters the intermediatespace between piston surface 5 and seating disc 29, on account of thequantity thereof, likewise leads to a closing off of the through passage44. The through passage 44, as a result, has no disadvantageous effecton the opening properties of the valve.

When there is a drop in pressure below the limiting value defined by thepiston surface 5 and the partial piston surface 41 of the drive head 28and defined by the force of the compression spring 8, the valve piston 3falls automatically back into its initial closed position, and residuesbetween piston surface 5 and seating disc 29 are conducted by way of thethrough passage 44 into the annular groove 42, which leads to a fasterpressure drop and thus to a faster closure of the valve. The oil residuecollected in the annular groove 42, in the closed condition according toFIG. 7, can drain off into the oil reservoir 13 through the gap 43 byway of the discharge port 12.

An alternative arrangement to the embodiment previously described isillustrated in FIG. 8. Here the annular groove 42 is provided in thecylinder bore 30 of the pump cylinder 31 at the level of the dischargeport 12, and the height of the annular groove corresponds to thediameter of the discharge port.

The through passage 44 opens, in this exemplary embodiment, into aradial bore 45 of the valve piston 3. This is at a distance b from thepiston surface 5, and dimension b is again chosen to be greater than thediameter c of the discharge port 12. For this reason, the radial bore 45is for the most part covered over radially by the bore wall of the pumpcylinder 31, except for a gap 43 to the annular groove 42, by way ofwhich gap 43 oil residues can drain away.

This radial bore 45 is also preferably disposed oppositely locatedrelative to the discharge port 12.

The mode of operation of the return valve 1 illustrated in FIG. 8corresponds to the embodiment described with reference to FIG. 7, inthat in both embodiments a hand-actuated displacement of the valvepiston 3 by way of the pulling part 37 can also take place. For thispurpose, there is provided an actuation rocker 37, pivotable about apivot pin 46 or the like mounted on the pump cylinder and having an arm36, one end of which acts on the pulling part 27.

The return valve 1 employed in the previously described embodiments canfind application in addition in hand or foot-operated pressing tools.

All features disclosed are pertinent to the invention. The disclosurecontent of the associated/attached priority documents (copy of the priorapplication) is hereby incorporated in its entirety in the disclosure ofthe application, for the additional purpose of incorporating features ofthese documents in claims of the present application.

What is claimed is:
 1. Hydraulic pressing device (2) comprising a stationary part (26) and a moving part (24), an inlet valve, an actuatable return valve (1) having an open position and a closed position, a return spring (10), and a hydraulic piston (9), the moving part (24) being moved relative to the stationary part (26) by means of the hydraulic piston (9) acted on by oil under pressure and being movable during a return stroke back into an initial position by means of the return spring (10), wherein the return stroke starts in dependence on a predetermined pressing pressure by actuation of the return valve (1), wherein the automatically actuating return valve (1) is maintained in the open position until the return stroke of the hydraulic piston (9) has been completed by the pressure of the oil during the return stroke.
 2. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a partial piston surface area which is effective in the closed condition and is configured with reference to a desired maximum pressure.
 3. Pressing device according to claim 2, further comprising a compression spring (8) and a cylinder (11), the return valve (1) being biased into the closed position by means of the compression spring (8), said valve piston (3) being accommodated in the cylinder (11), said cylinder (11) including a discharge port (12) to an oil reservoir (13), wherein the discharge port (12) is configured to open in the course of movement of the valve piston (3) into the open position.
 4. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3), said pressing device further comprising a cylinder having a wall, wherein a relief bore (14) penetrates the cylinder wall and is provided at a rear portion of the valve piston (3).
 5. Pressing device according to claim 3, wherein the return valve (1) is formed as a valve piston (3), wherein the valve piston (3) is movable into the open position by manual actuation.
 6. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3), said pressing device further comprising a pulling part (27) and a cylinder (11), wherein said pulling part (27) is connected to the valve piston (3) and passes through the cylinder (11).
 7. Pressing device according to claim 6, further comprising an actuating rocker (37), wherein said pulling part (27) is moveable manually by means of the actuating rocker (37).
 8. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a pressure face, wherein the valve piston (3), rearwardly of its pressure face, is generally pot-shaped.
 9. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a drive nose, said pressing device further comprising a pulling part (27), said pulling part (27) comprising a drive head (28) wherein said drive head (28) stands in engagement with the drive nose of the valve piston (3).
 10. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3), said pressing device further comprising a compression spring (8) and a pulling part (27), wherein the compression spring (8) acts on the valve piston (3) by way of the pulling part (27).
 11. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a pot wall (32) and a drive nose, wherein the drive nose is an expanding ring (33) which is disposed in the pot wall (32) of the valve piston (3).
 12. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a pot wall (32) and a drive nose, wherein the drive nose is a radial collar (40) formed in the pot wall (32) of the valve piston (3), said pressing device further comprising a pulling part (27), wherein the collar (40) is configured to generally center the pulling part (27) in the valve piston (3).
 13. Pressing device according to claim 1, said pressing device further comprising a cylindrical pulling part (27), said cylindrical pulling part (27) including a drive head (28) formed as a circumferential flange on the cylindrical pulling part (27).
 14. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3), said pressing device further comprising a pulling part (27) having a spigot portion (40) of reduced diameter and a drive head (28), wherein the pulling part (27) acts upon the valve piston (3) by way of the spigot portion (40), wherein the spigot portion (40) is centrally disposed on the drive head (28).
 15. Pressing device according to claim 1, said pressing device further comprising a pulling part (27) having a drive head (28), wherein the drive head (28) defines an active piston surface when the return valve (1) is in the open position.
 16. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having an active valve piston surface (5), said pressing device further comprising a pulling part (27) having a drive head (28), wherein the drive head (28), when the return valve (1) is in the open position, defines a partial piston surface (41) integrated with the active valve piston surface (5) of the valve piston (3).
 17. Pressing device according to claim 1, further comprising a pulling part (27) having a drive head (28), wherein the drive head (28), in the closed condition, defines a partial piston surface area which is configured with reference to a desired maximum pressure.
 18. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a closed circumference, said closed circumference having a height (b), said pressing device further comprising a cylinder (11) which includes a discharge port (12) to an oil reservoir (13), said discharge port (12) having a diameter (c), wherein the diameter (c) of the discharge port (12) is smaller than the height (b) of a closed circumference of the valve piston (3).
 19. Pressing device according to claim 3, wherein the return valve (1) is formed as a valve piston (3) having a valve piston surface (5) and an outer sleeve, wherein an annular groove (42) open to the outer sleeve of the valve piston (3) is provided rearwardly of the valve piston surface (5).
 20. Pressing device according to claim 19, wherein the return valve (1) is formed as a valve piston (3) having a valve piston surface (5), said pressing device further comprising a cylinder (11) which includes a discharge port (12) to an oil reservoir (13), said discharge port (12) having a diameter (c), wherein a distance between the valve piston surface (5) and the annular groove (42) is greater than the diameter (c) of the discharge port (12).
 21. Pressing device according to claim 20, wherein an axially aligned through passage (44) extends from the valve piston surface (5) for connecting the valve piston surface (5) with the annular groove (42).
 22. Pressing device according to claim 21, wherein the valve (1) has an oil inlet bore (7), and the oil inlet bore (7) has a diameter, said through passage (44) having a diameter, wherein the diameter of the through passage (44) is smaller than the diameter of an oil inlet bore (7) of the valve (1).
 23. Pressing device according to claim 1, wherein the return valve (1) is formed as a valve piston (3) having a valve piston surface (5) and an outer wall (32), said pressing device further comprising a cylinder (11) having a cylinder bore (30), wherein an annular groove (42) is provided in the outer wall (32) of the valve piston (3), for a substantially horizontal disposition of the cylinder bore (30).
 24. Pressing device according to claim 23, wherein the annular groove (42) is provided in the cylinder bore (30) and the valve piston (3) has an associated radial bore (45). 